As is well known, prospecting for minerals of commercial or other value (including but not limited to hydrocarbons in liquid or gaseous form; water e.g. in aquifers; and various solids used e.g. as fuels, ores or in manufacturing) is economically an extremely important activity. For various reasons those wishing to extract such minerals from below the surface of the ground or the floor of an ocean need to acquire as much information as possible about both the potential commercial worth of the minerals in a geological formation and also any difficulties that may arise in the extraction of the minerals to surface locations at which they may be used.
For this reason over many decades techniques of logging of subterranean formations have developed for the purpose of establishing, with as much accuracy as possible, information as outlined above both before mineral extraction activities commence and also, increasingly frequently, while they are taking place.
Broadly stated, logging involves inserting a logging tool including a section sometimes called a “sonde” into a borehole or other feature penetrating a formation under investigation; and in the majority of cases using the sonde to energize the material of the rock, etc., surrounding the borehole in some way. The sonde or another tool associated with it that is capable of detecting energy is intended then to receive emitted energy that has passed through the various components in the rock before being recorded by the logging tool.
Such passage of the energy alters its character. Knowledge of the attributes of the emitted energy and that detected after passage through the rock may reveal considerable information about the chemistry, concentration, quantity and a host of other characteristics of minerals in the vicinity of the borehole, as well as geological aspects that influence the ease with which the target mineral material may be extracted to a surface location.
Logging techniques are employed throughout the mining industry, and also in particular in the oil and gas industries. The disclosed subject matter is of benefit in logging activities potentially in all kinds of mining and especially in the logging of reserves of oil and gas.
In the logging of oil, coal and gas fields (including fields combined with rock types such as shales) specific problems can arise. Broadly stated, this is because it is helpful to consider a geological formation, surrounding a borehole, that typically is porous and that contains a hydrocarbon-containing fluid such as oil or gas or (commonly) a mixture of fluids only one component of which is of commercial value.
This leads to various complications associated with determining geological attributes of the oil or gas field in question. In consequence, a wide variety of logging methods has been developed over the years. The logging techniques exploit physical and chemical properties of a formation usually through the use of a logging tool or sonde that as outlined above is lowered into a borehole (that typically is, but need not be, a wellbore) formed in the formation by drilling.
Typically, as noted, the tool sends energy into the formation and detects the energy returned to it that has been altered in some way by the formation. The nature of any such alteration can be processed into electrical signals that are then used to generate logs (i.e. graphical or tabular representations containing much data about the formation in question).
One specific aspect of logging that has developed in recent years is the practice of logging a borehole while it is being drilled. This technique, which is sometimes referred to as “logging while drilling” (or “LWD”), can for example permit the direction of drilling to be altered such that the borehole extends for a maximal distance through a region of the geological formation that is believed to be rich in minerals of value. Another example of an LWD technique involves using the log data to help steer the drill head that forms the borehole so as to avoid regions of rock that are likely to be problematic in terms of borehole stability or other adverse factors in mineral recovery.
The disclosed subject matter is potentially of use in, and pertains to, all aspects of logging as described herein.
One characteristic of formations surrounding boreholes that is of potential interest to petrophysicists and others charged with the task of assessing downhole environments is the relative permittivity or dielectric constant of the rock formation.
One reason this quantity is of interest is that its value is distinctive depending on the fluid contained in the pores of the rock. This is of potential benefit in determining whether the rock pores contain water, oil, gas or a mixture of these fluids.
In particular it is known that the dielectric constant of liquid hydrocarbons, especially oil, is in the range 1-3; that of rock is in the range 4-10; and that of water is in the range 56-80. An accurate measure of dielectric constant can provide a direct indication of the make-up of fluids (including fluid mixtures) in the rock pores. Different mixtures of fluids can influence the dielectric constant to a variable extent; and the measured value also may be influenced by the rock type.
This is an advantage of a dielectric constant measurement over for example a resistivity measurement that, while widely used in logging technologies, can be ambiguous when seeking to distinguish between rock-borne water and rock-borne hydrocarbons. This is particularly the case when the water in the formation is of low or zero salinity, although the problems of resistivity measurement ambiguity diminish with increasing salinity of any water in the pores of the rock.
Despite the apparent advantages of dielectric constant measurements they have not been widely used in logging techniques. It is believed that the primary reason for this is a lack of faith on the part of analysts and petrophysicists in the reliability of subterranean dielectric constant measurements.
This in turn is because existing equipment for measuring dielectric constant values in downhole locations (i.e. locations in boreholes as referred to above) is known to suffer from poor depth of penetration characteristics. As a result the emitted energy passes only a relatively short distance into the rock surrounding a borehole. As a consequence the dielectric constant measurement does not give an acceptably comprehensive picture of the conditions prevailing in the rock.
Known designs of logging tool used for measuring dielectric constant employ typically a transmitter of electromagnetic energy and a receiver antenna that is spaced from the transmitter along the logging tool (which in nearly all cases is an elongate cylinder of perhaps about 75 mm diameter and several meters in length). Although the emitted energy is intended to pass into the rock before returning to the receiver antenna in practice a significant percentage of it travels parallel to the logging tool without appreciably penetrating the rock. That which passes into the rock is usually of limited use because it returns to the receiver antenna as reflected energy. The point of reflection is not known and therefore it is not known how far the energy has travelled after leaving the transmitter antenna before being received at the receiver antenna.
The energy that travels parallel to the logging tool gives an accurate indication of the dielectric constant of the materials through which it has passed. Such a signal however is essentially wasted as it does not imply on receipt at the antenna any information about the permittivity of the rock at any location except very close to the borehole. The returned energy therefore may convey as much information about the borehole surface layer which may be composed of mudcake lining the borehole and/or near-borehole fluid mixing and/or borehole rugosity as information about rock-borne fluids. Since mudcake forms from chemicals intentionally introduced into the borehole during drilling, information about its make-up is of limited benefit.
Moreover the signals derived from such directly transmitted energy and signals derived from energy that has passed through rock can interfere unacceptably with one another, distorting the output of the receiver antenna and making it hard to process and interpret electronically.
A further problem with existing antenna-based dielectric constant measuring logging tools is that they suffer from unwanted reflections of emitted signals. Such reflections can occur with respect to the opposite side of the logging tool to that at which investigations are required. The reflected signals too tend to interfere destructively with the desired signals.
U.S. Pat. No. 7,376,514 describes variants on the basic antenna type of logging tool. This publication observes that the dielectric constant in fluid-filled rock can be generated as a complex number. U.S. Pat. No. 7,376,514 describes the measuring of the complex dielectric constant value at three distinct frequencies, and an associated processing technique.
It is believed that an antenna used to couple electromagnetic energy at three separate frequencies for the purpose of determining complex dielectric constant values can only function by resonating relatively weakly at at least two of the frequencies. This conclusion derives from the well-known fact that an antenna used to detect electromagnetic radiation is tuned to couple strongly only at a single frequency, with less efficient detection at harmonics of the tuned frequency and considerably less efficient detection (or, more probably, non-existent detection) at non-harmonic frequencies.
In view of the foregoing there is a need for improvements in the detection of dielectric constant values of rock formations surrounding subterranean boreholes.